msm: dma: add completion.h header

At some point this was exposed (not sure how),

linux-2.6/arch/arm/mach-msm/dma.c:92: error: field 'complete' has incomplete type
linux-2.6/arch/arm/mach-msm/dma.c: In function 'dmov_exec_cmdptr_complete_func':
linux-2.6/arch/arm/mach-msm/dma.c:108: error: implicit declaration of function 'complete'
linux-2.6/arch/arm/mach-msm/dma.c: In function 'msm_dmov_exec_cmd':
linux-2.6/arch/arm/mach-msm/dma.c:120: error: implicit declaration of function 'init_completion'
linux-2.6/arch/arm/mach-msm/dma.c:123: error: implicit declaration of function 'wait_for_completion'

and the fix is just to add the header.

Signed-off-by: Daniel Walker <dwalker@codeaurora.org>